Immunoglobulin heavy chain variable region gene replacement as a mechanism for receptor revision in rheumatoid arthritis synovial tissue B lymphocytes

Mature B cells can alter their antibody repertoires by several mechanisms, including immunoglobulin heavy chain variable region (V-H) replacement. This process changes the antigen combining site by replacing a portion of the original V-H/diversity/heavy chain joining region (V-H-DJ(H)) rearrangement with a corresponding portion of a new V-H segment. This exchange can involve cryptic heptamer-like sequences embedded in the coding regions of V-H genes. While studying the B lymphocytes that expand in the synovial tissues of patients with rheumatoid arthritis (RA), clones with V(H)DJ(H) variants that were apparently generated by V-H replacement were identified with surprising frequency (similar to8%). Examples of multiple independent V-H replacement events occurring in distinct progeny clones were also identified. These secondary V-H rearrangements were documented at both the cDNA and genomic DNA levels and involved several heptamer-like sequences at four distinct locations within V-H (three sites in framework region 3 and one in complementarity determining region 2). The identification of blunt-ended double-stranded DNA breaks at the embedded heptamers and the demonstration of recombinase activating gene (RAG) expression suggested that these rearrangements could occur in the synovial tissues, presumably in pseudo-germinal centers, and that they could be mediated by RAG in a recognition signal sequence-specific manner. The presence of V-H mutations in the clones that had undergone replacement indicated that these B cells were immunocompetent and could receive and respond to diversification signals. A relationship between these secondary V-H gene rearrangements and the autoimmunity characteristic of RA should be considered.